Date:   Wed, 07 Dec 2022 11:08:08 +0100
From:   Finn Behrens <fin@...ntec.com>
To:     ojeda@...nel.org
Cc:     Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH v1] rust: alloc: remove the `borrow` module (`ToOwned`,
 `Cow`)



On 6 Dec 2022, at 2:05, ojeda@...nel.org wrote:

> From: Miguel Ojeda <ojeda@...nel.org>
>
> The `Cow` type [1] requires that its generic parameter type implements
> the `ToOwned` trait [2], which provides a method to create owned data
> from borrowed data, usually by cloning.
>
> However, it is infallible, and thus in most cases it is not useful for
> the kernel. [3]
>
> Therefore, introduce `cfg(no_borrow)` to remove the `borrow` module
> (which contains `ToOwned` and `Cow`) from `alloc`.
Maybe in the future, if Cow is helpful, we could create a TryToOwned trait (and then probably TryCow), but for now this looks good.

>
> Link: https://doc.rust-lang.org/alloc/borrow/enum.Cow.html [1]
> Link: https://doc.rust-lang.org/alloc/borrow/trait.ToOwned.html [2]
> Link: https://lore.kernel.org/rust-for-linux/20221204103153.117675b1@GaryWorkstation/ [3]
> Cc: Gary Guo <gary@...yguo.net>
> Cc: Wedson Almeida Filho <wedsonaf@...il.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
Reviewed-by: Finn Behrens <fin@...ntec.com>

Regards,
Finn

> ---
>  rust/Makefile         |   1 +
>  rust/alloc/borrow.rs  | 498 ------------------------------------------
>  rust/alloc/lib.rs     |   3 +-
>  rust/alloc/vec/mod.rs |   3 +
>  4 files changed, 6 insertions(+), 499 deletions(-)
>  delete mode 100644 rust/alloc/borrow.rs
>
> diff --git a/rust/Makefile b/rust/Makefile
> index 7700d3853404..208b520fcbea 100644
> --- a/rust/Makefile
> +++ b/rust/Makefile
> @@ -44,6 +44,7 @@ core-cfgs = \
>      --cfg no_fp_fmt_parse
>
>  alloc-cfgs = \
> +    --cfg no_borrow \
>      --cfg no_fmt \
>      --cfg no_global_oom_handling \
>      --cfg no_macros \
> diff --git a/rust/alloc/borrow.rs b/rust/alloc/borrow.rs
> deleted file mode 100644
> index dde4957200d4..000000000000
> --- a/rust/alloc/borrow.rs
> +++ /dev/null
> @@ -1,498 +0,0 @@
> -// SPDX-License-Identifier: Apache-2.0 OR MIT
> -
> -//! A module for working with borrowed data.
> -
> -#![stable(feature = "rust1", since = "1.0.0")]
> -
> -use core::cmp::Ordering;
> -use core::hash::{Hash, Hasher};
> -use core::ops::Deref;
> -#[cfg(not(no_global_oom_handling))]
> -use core::ops::{Add, AddAssign};
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -pub use core::borrow::{Borrow, BorrowMut};
> -
> -use core::fmt;
> -#[cfg(not(no_global_oom_handling))]
> -use crate::string::String;
> -
> -use Cow::*;
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
> -where
> -    B: ToOwned,
> -    <B as ToOwned>::Owned: 'a,
> -{
> -    fn borrow(&self) -> &B {
> -        &**self
> -    }
> -}
> -
> -/// A generalization of `Clone` to borrowed data.
> -///
> -/// Some types make it possible to go from borrowed to owned, usually by
> -/// implementing the `Clone` trait. But `Clone` works only for going from `&T`
> -/// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
> -/// from any borrow of a given type.
> -#[cfg_attr(not(test), rustc_diagnostic_item = "ToOwned")]
> -#[stable(feature = "rust1", since = "1.0.0")]
> -pub trait ToOwned {
> -    /// The resulting type after obtaining ownership.
> -    #[stable(feature = "rust1", since = "1.0.0")]
> -    type Owned: Borrow<Self>;
> -
> -    /// Creates owned data from borrowed data, usually by cloning.
> -    ///
> -    /// # Examples
> -    ///
> -    /// Basic usage:
> -    ///
> -    /// ```
> -    /// let s: &str = "a";
> -    /// let ss: String = s.to_owned();
> -    ///
> -    /// let v: &[i32] = &[1, 2];
> -    /// let vv: Vec<i32> = v.to_owned();
> -    /// ```
> -    #[stable(feature = "rust1", since = "1.0.0")]
> -    #[must_use = "cloning is often expensive and is not expected to have side effects"]
> -    fn to_owned(&self) -> Self::Owned;
> -
> -    /// Uses borrowed data to replace owned data, usually by cloning.
> -    ///
> -    /// This is borrow-generalized version of `Clone::clone_from`.
> -    ///
> -    /// # Examples
> -    ///
> -    /// Basic usage:
> -    ///
> -    /// ```
> -    /// # #![feature(toowned_clone_into)]
> -    /// let mut s: String = String::new();
> -    /// "hello".clone_into(&mut s);
> -    ///
> -    /// let mut v: Vec<i32> = Vec::new();
> -    /// [1, 2][..].clone_into(&mut v);
> -    /// ```
> -    #[unstable(feature = "toowned_clone_into", reason = "recently added", issue = "41263")]
> -    fn clone_into(&self, target: &mut Self::Owned) {
> -        *target = self.to_owned();
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<T> ToOwned for T
> -where
> -    T: Clone,
> -{
> -    type Owned = T;
> -    fn to_owned(&self) -> T {
> -        self.clone()
> -    }
> -
> -    fn clone_into(&self, target: &mut T) {
> -        target.clone_from(self);
> -    }
> -}
> -
> -/// A clone-on-write smart pointer.
> -///
> -/// The type `Cow` is a smart pointer providing clone-on-write functionality: it
> -/// can enclose and provide immutable access to borrowed data, and clone the
> -/// data lazily when mutation or ownership is required. The type is designed to
> -/// work with general borrowed data via the `Borrow` trait.
> -///
> -/// `Cow` implements `Deref`, which means that you can call
> -/// non-mutating methods directly on the data it encloses. If mutation
> -/// is desired, `to_mut` will obtain a mutable reference to an owned
> -/// value, cloning if necessary.
> -///
> -/// If you need reference-counting pointers, note that
> -/// [`Rc::make_mut`][crate::rc::Rc::make_mut] and
> -/// [`Arc::make_mut`][crate::sync::Arc::make_mut] can provide clone-on-write
> -/// functionality as well.
> -///
> -/// # Examples
> -///
> -/// ```
> -/// use std::borrow::Cow;
> -///
> -/// fn abs_all(input: &mut Cow<[i32]>) {
> -///     for i in 0..input.len() {
> -///         let v = input[i];
> -///         if v < 0 {
> -///             // Clones into a vector if not already owned.
> -///             input.to_mut()[i] = -v;
> -///         }
> -///     }
> -/// }
> -///
> -/// // No clone occurs because `input` doesn't need to be mutated.
> -/// let slice = [0, 1, 2];
> -/// let mut input = Cow::from(&slice[..]);
> -/// abs_all(&mut input);
> -///
> -/// // Clone occurs because `input` needs to be mutated.
> -/// let slice = [-1, 0, 1];
> -/// let mut input = Cow::from(&slice[..]);
> -/// abs_all(&mut input);
> -///
> -/// // No clone occurs because `input` is already owned.
> -/// let mut input = Cow::from(vec![-1, 0, 1]);
> -/// abs_all(&mut input);
> -/// ```
> -///
> -/// Another example showing how to keep `Cow` in a struct:
> -///
> -/// ```
> -/// use std::borrow::Cow;
> -///
> -/// struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
> -///     values: Cow<'a, [X]>,
> -/// }
> -///
> -/// impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
> -///     fn new(v: Cow<'a, [X]>) -> Self {
> -///         Items { values: v }
> -///     }
> -/// }
> -///
> -/// // Creates a container from borrowed values of a slice
> -/// let readonly = [1, 2];
> -/// let borrowed = Items::new((&readonly[..]).into());
> -/// match borrowed {
> -///     Items { values: Cow::Borrowed(b) } => println!("borrowed {b:?}"),
> -///     _ => panic!("expect borrowed value"),
> -/// }
> -///
> -/// let mut clone_on_write = borrowed;
> -/// // Mutates the data from slice into owned vec and pushes a new value on top
> -/// clone_on_write.values.to_mut().push(3);
> -/// println!("clone_on_write = {:?}", clone_on_write.values);
> -///
> -/// // The data was mutated. Let's check it out.
> -/// match clone_on_write {
> -///     Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
> -///     _ => panic!("expect owned data"),
> -/// }
> -/// ```
> -#[stable(feature = "rust1", since = "1.0.0")]
> -#[cfg_attr(not(test), rustc_diagnostic_item = "Cow")]
> -pub enum Cow<'a, B: ?Sized + 'a>
> -where
> -    B: ToOwned,
> -{
> -    /// Borrowed data.
> -    #[stable(feature = "rust1", since = "1.0.0")]
> -    Borrowed(#[stable(feature = "rust1", since = "1.0.0")] &'a B),
> -
> -    /// Owned data.
> -    #[stable(feature = "rust1", since = "1.0.0")]
> -    Owned(#[stable(feature = "rust1", since = "1.0.0")] <B as ToOwned>::Owned),
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<B: ?Sized + ToOwned> Clone for Cow<'_, B> {
> -    fn clone(&self) -> Self {
> -        match *self {
> -            Borrowed(b) => Borrowed(b),
> -            Owned(ref o) => {
> -                let b: &B = o.borrow();
> -                Owned(b.to_owned())
> -            }
> -        }
> -    }
> -
> -    fn clone_from(&mut self, source: &Self) {
> -        match (self, source) {
> -            (&mut Owned(ref mut dest), &Owned(ref o)) => o.borrow().clone_into(dest),
> -            (t, s) => *t = s.clone(),
> -        }
> -    }
> -}
> -
> -impl<B: ?Sized + ToOwned> Cow<'_, B> {
> -    /// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
> -    ///
> -    /// # Examples
> -    ///
> -    /// ```
> -    /// #![feature(cow_is_borrowed)]
> -    /// use std::borrow::Cow;
> -    ///
> -    /// let cow = Cow::Borrowed("moo");
> -    /// assert!(cow.is_borrowed());
> -    ///
> -    /// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
> -    /// assert!(!bull.is_borrowed());
> -    /// ```
> -    #[unstable(feature = "cow_is_borrowed", issue = "65143")]
> -    #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
> -    pub const fn is_borrowed(&self) -> bool {
> -        match *self {
> -            Borrowed(_) => true,
> -            Owned(_) => false,
> -        }
> -    }
> -
> -    /// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
> -    ///
> -    /// # Examples
> -    ///
> -    /// ```
> -    /// #![feature(cow_is_borrowed)]
> -    /// use std::borrow::Cow;
> -    ///
> -    /// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
> -    /// assert!(cow.is_owned());
> -    ///
> -    /// let bull = Cow::Borrowed("...moo?");
> -    /// assert!(!bull.is_owned());
> -    /// ```
> -    #[unstable(feature = "cow_is_borrowed", issue = "65143")]
> -    #[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
> -    pub const fn is_owned(&self) -> bool {
> -        !self.is_borrowed()
> -    }
> -
> -    /// Acquires a mutable reference to the owned form of the data.
> -    ///
> -    /// Clones the data if it is not already owned.
> -    ///
> -    /// # Examples
> -    ///
> -    /// ```
> -    /// use std::borrow::Cow;
> -    ///
> -    /// let mut cow = Cow::Borrowed("foo");
> -    /// cow.to_mut().make_ascii_uppercase();
> -    ///
> -    /// assert_eq!(
> -    ///   cow,
> -    ///   Cow::Owned(String::from("FOO")) as Cow<str>
> -    /// );
> -    /// ```
> -    #[stable(feature = "rust1", since = "1.0.0")]
> -    pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
> -        match *self {
> -            Borrowed(borrowed) => {
> -                *self = Owned(borrowed.to_owned());
> -                match *self {
> -                    Borrowed(..) => unreachable!(),
> -                    Owned(ref mut owned) => owned,
> -                }
> -            }
> -            Owned(ref mut owned) => owned,
> -        }
> -    }
> -
> -    /// Extracts the owned data.
> -    ///
> -    /// Clones the data if it is not already owned.
> -    ///
> -    /// # Examples
> -    ///
> -    /// Calling `into_owned` on a `Cow::Borrowed` returns a clone of the borrowed data:
> -    ///
> -    /// ```
> -    /// use std::borrow::Cow;
> -    ///
> -    /// let s = "Hello world!";
> -    /// let cow = Cow::Borrowed(s);
> -    ///
> -    /// assert_eq!(
> -    ///   cow.into_owned(),
> -    ///   String::from(s)
> -    /// );
> -    /// ```
> -    ///
> -    /// Calling `into_owned` on a `Cow::Owned` returns the owned data. The data is moved out of the
> -    /// `Cow` without being cloned.
> -    ///
> -    /// ```
> -    /// use std::borrow::Cow;
> -    ///
> -    /// let s = "Hello world!";
> -    /// let cow: Cow<str> = Cow::Owned(String::from(s));
> -    ///
> -    /// assert_eq!(
> -    ///   cow.into_owned(),
> -    ///   String::from(s)
> -    /// );
> -    /// ```
> -    #[stable(feature = "rust1", since = "1.0.0")]
> -    pub fn into_owned(self) -> <B as ToOwned>::Owned {
> -        match self {
> -            Borrowed(borrowed) => borrowed.to_owned(),
> -            Owned(owned) => owned,
> -        }
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
> -impl<B: ?Sized + ToOwned> const Deref for Cow<'_, B>
> -where
> -    B::Owned: ~const Borrow<B>,
> -{
> -    type Target = B;
> -
> -    fn deref(&self) -> &B {
> -        match *self {
> -            Borrowed(borrowed) => borrowed,
> -            Owned(ref owned) => owned.borrow(),
> -        }
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<B: ?Sized> Ord for Cow<'_, B>
> -where
> -    B: Ord + ToOwned,
> -{
> -    #[inline]
> -    fn cmp(&self, other: &Self) -> Ordering {
> -        Ord::cmp(&**self, &**other)
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
> -where
> -    B: PartialEq<C> + ToOwned,
> -    C: ToOwned,
> -{
> -    #[inline]
> -    fn eq(&self, other: &Cow<'b, C>) -> bool {
> -        PartialEq::eq(&**self, &**other)
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
> -where
> -    B: PartialOrd + ToOwned,
> -{
> -    #[inline]
> -    fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
> -        PartialOrd::partial_cmp(&**self, &**other)
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<B: ?Sized> fmt::Debug for Cow<'_, B>
> -where
> -    B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
> -{
> -    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> -        match *self {
> -            Borrowed(ref b) => fmt::Debug::fmt(b, f),
> -            Owned(ref o) => fmt::Debug::fmt(o, f),
> -        }
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<B: ?Sized> fmt::Display for Cow<'_, B>
> -where
> -    B: fmt::Display + ToOwned<Owned: fmt::Display>,
> -{
> -    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> -        match *self {
> -            Borrowed(ref b) => fmt::Display::fmt(b, f),
> -            Owned(ref o) => fmt::Display::fmt(o, f),
> -        }
> -    }
> -}
> -
> -#[stable(feature = "default", since = "1.11.0")]
> -impl<B: ?Sized> Default for Cow<'_, B>
> -where
> -    B: ToOwned<Owned: Default>,
> -{
> -    /// Creates an owned Cow<'a, B> with the default value for the contained owned value.
> -    fn default() -> Self {
> -        Owned(<B as ToOwned>::Owned::default())
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<B: ?Sized> Hash for Cow<'_, B>
> -where
> -    B: Hash + ToOwned,
> -{
> -    #[inline]
> -    fn hash<H: Hasher>(&self, state: &mut H) {
> -        Hash::hash(&**self, state)
> -    }
> -}
> -
> -#[stable(feature = "rust1", since = "1.0.0")]
> -impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
> -    fn as_ref(&self) -> &T {
> -        self
> -    }
> -}
> -
> -#[cfg(not(no_global_oom_handling))]
> -#[stable(feature = "cow_add", since = "1.14.0")]
> -impl<'a> Add<&'a str> for Cow<'a, str> {
> -    type Output = Cow<'a, str>;
> -
> -    #[inline]
> -    fn add(mut self, rhs: &'a str) -> Self::Output {
> -        self += rhs;
> -        self
> -    }
> -}
> -
> -#[cfg(not(no_global_oom_handling))]
> -#[stable(feature = "cow_add", since = "1.14.0")]
> -impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
> -    type Output = Cow<'a, str>;
> -
> -    #[inline]
> -    fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
> -        self += rhs;
> -        self
> -    }
> -}
> -
> -#[cfg(not(no_global_oom_handling))]
> -#[stable(feature = "cow_add", since = "1.14.0")]
> -impl<'a> AddAssign<&'a str> for Cow<'a, str> {
> -    fn add_assign(&mut self, rhs: &'a str) {
> -        if self.is_empty() {
> -            *self = Cow::Borrowed(rhs)
> -        } else if !rhs.is_empty() {
> -            if let Cow::Borrowed(lhs) = *self {
> -                let mut s = String::with_capacity(lhs.len() + rhs.len());
> -                s.push_str(lhs);
> -                *self = Cow::Owned(s);
> -            }
> -            self.to_mut().push_str(rhs);
> -        }
> -    }
> -}
> -
> -#[cfg(not(no_global_oom_handling))]
> -#[stable(feature = "cow_add", since = "1.14.0")]
> -impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
> -    fn add_assign(&mut self, rhs: Cow<'a, str>) {
> -        if self.is_empty() {
> -            *self = rhs
> -        } else if !rhs.is_empty() {
> -            if let Cow::Borrowed(lhs) = *self {
> -                let mut s = String::with_capacity(lhs.len() + rhs.len());
> -                s.push_str(lhs);
> -                *self = Cow::Owned(s);
> -            }
> -            self.to_mut().push_str(&rhs);
> -        }
> -    }
> -}
> diff --git a/rust/alloc/lib.rs b/rust/alloc/lib.rs
> index 233bcd5e4654..3aebf83c9967 100644
> --- a/rust/alloc/lib.rs
> +++ b/rust/alloc/lib.rs
> @@ -100,7 +100,7 @@
>  #![cfg_attr(not(no_global_oom_handling), feature(const_alloc_error))]
>  #![feature(const_box)]
>  #![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
> -#![feature(const_cow_is_borrowed)]
> +#![cfg_attr(not(no_borrow), feature(const_cow_is_borrowed))]
>  #![feature(const_convert)]
>  #![feature(const_size_of_val)]
>  #![feature(const_align_of_val)]
> @@ -215,6 +215,7 @@ pub mod boxed;
>  mod boxed {
>      pub use std::boxed::Box;
>  }
> +#[cfg(not(no_borrow))]
>  pub mod borrow;
>  pub mod collections;
>  #[cfg(not(no_global_oom_handling))]
> diff --git a/rust/alloc/vec/mod.rs b/rust/alloc/vec/mod.rs
> index 540787804cc2..ae0085e1231f 100644
> --- a/rust/alloc/vec/mod.rs
> +++ b/rust/alloc/vec/mod.rs
> @@ -72,6 +72,7 @@ use core::ptr::{self, NonNull};
>  use core::slice::{self, SliceIndex};
>
>  use crate::alloc::{Allocator, Global};
> +#[cfg(not(no_borrow))]
>  use crate::borrow::{Cow, ToOwned};
>  use crate::boxed::Box;
>  use crate::collections::TryReserveError;
> @@ -94,6 +95,7 @@ pub use self::drain::Drain;
>
>  mod drain;
>
> +#[cfg(not(no_borrow))]
>  #[cfg(not(no_global_oom_handling))]
>  mod cow;
>
> @@ -3014,6 +3016,7 @@ impl<T, const N: usize> From<[T; N]> for Vec<T> {
>      }
>  }
>
> +#[cfg(not(no_borrow))]
>  #[stable(feature = "vec_from_cow_slice", since = "1.14.0")]
>  impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
>  where
>
> base-commit: 76dcd734eca23168cb008912c0f69ff408905235
> -- 
> 2.38.1

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